Why does Paxil (paroxetine) make me jittery and eat more?

5-HT receptors. SSRI's like Paroxetine both activate or inhibit multiple serotinergic (5-HT), receptors. At least 4 of them regulate appetite and some anxiety and agitation (jitters). Side effects affect people somewhat differently. Some get a significant desirable antidepressant or other therapeutic goals, and some others experience side effects such as agitation or increased appetite and other undesirable SE's.